It is not unusual in the formation of compositions containing chromic acid, or its equivalent, that are used in coating metal substrates, to bear in mind the pH of the coating composition. For example, in the chromate conversion coating art such may be the case since these coatings are ostensibly developed for application at least to aluminum surfaces where they will attack the surface during film formation. Such chromate conversion coatings therefor contain acidic substances to enhance the attack on a substrate metal; and they further contain substances such as those supplying fluoride ions that may be termed accelerators. Such ions thus augment film formation. In U.S. Pat. No. 3,113,051 a chromate conversion coating for aluminum surfaces has been disclosed and it is further taught therein that the coating composition should have a pH for best coating formulation of between about 1.3-2.2.
In other coating compositions containing chromic acid or its equivalent, but which are not of the nature of conversion coatings, pH may also be important. For example, in U.S. Pat. No. 3,630,789 a treating solution that can be free from ions such as fluoride ions and also strong acids for substrate metal attack, is nevertheless formulated for application to metal substrates. Further, it is formulated with careful control to maintain the treating solution pH between about 1.8-5 to prevent the reaction of composition ingredients before application, while maintaining a bath that will effectively treat base metals.
In chromic-acid-containing compositions, also containing reducing agent for the chromic acid, the presence of ammonia has been disclosed to have special usefulness. Thus, in U.S. Pat. No. 2,911,332, ammonia has been added, for example to improve corrosion resistance of subsequent coatings on tin-plated steel. After the addition of ammonia the resulting composition is ready for use, even though the coating bath pH may be slightly alkaline.
In the continuing development of the coating compositions that are treating solutions, and which may also be referred to as bonding compositions as in U.S. Pat. No. 3,382,081, one development includes formulation with pulverulent metal, especially pulverulent zinc. Thus, U.S. Pat. No. 3,671,331 discloses employing finely divided zinc most particularly in bonding coatings, i.e., compositions containing hexavalent chromium providing substance, such as chromic acid, and an agent for reducing the hexavalent chromium provided by the chromic acid. Further, U.S. Pat. No. 3,687,738 discloses the development of a coating composition that may contain, as principle ingredients, pulverulent zinc plus chromic acid. In the formulation of such compositions it would be most desirable to provide a coating composition having extended bath stability. In this regard it would be most especially desirable that such extended bath stability include freedom from gelation as this phenomenon is virtually irreversible.